Electromechanical transducers are machines, which convert electrical energy into mechanical energy or vice versa. An electric motor is a widely used electromechanical transducer that converts electrical energy into mechanical energy using magnetic field linkage. An electric generator is an electromechanical transducer that converts mechanical energy into electrical energy also using a magnetic field linkage.
An electromechanical transducer comprises a stator and a rotor. The stator is an assembly, which represents the stationary part of an electromechanical transducer. The rotor is an assembly, which represents the moving part of an electromechanical transducer.
In order to realize a magnetic field linkage permanent magnets may be used in particular for a rotor of an electromechanical transducer. In recent years, especially since the introduction of rare-earth magnetic materials, permanent magnet (PM) electromechanical transducers have become popular since they eliminate the need for commutator and brushes, which are commonly used with conventional Direct Current (DC) electromechanical transducer. The absence of an external electrical rotor excitation eliminates losses on the rotor and makes permanent magnet electromechanical transducers more efficient. Further, the brushless design of a PM electro-mechanical transducer allows conductor coils to be located exclusively in the stationary stator. In this respect it is mentioned that non-PM electromechanical transducers, which are equipped with commutators and brushes, are susceptible to significantly higher maintenance costs.
PM electromechanical transducers are also known for their durability, controllability, and absence of electrical sparking. Thanks to their advantages the PM electromechanical transducers are widely used in many applications such as electric vehicles (electromechanical transducer is a motor) or in power generation systems (electromechanical transducer is a generator) such as for instance a wind turbine.
One technical problem of PM electromechanical transducers is cogging torque. Cogging torque is produced between the rotor mounted permanent magnets and the stator due to a meeting of “sharp edges” of the rotor magnets and stator coils when the PM electromechanical transducer is in operation. Cogging torque is an undesired effect that contributes to an output ripple (also called torque ripple), to vibrations, and to noise in an electromechanical transducer.
It is known that skewing of the rotor magnets can reduce or nearly eliminate cogging torque in permanent magnet electric machines. For instance U.S. Pat. No. 6,867,524 B2 discloses a permanent magnet motor comprising a rotor having at least three segments. Each of the three segments is formed sequentially adjacent and aligned along an axis of the rotor. Each segment has at least one pair of permanent magnets disposed at a substantially equal interval in a peripheral direction of the rotor. First and second segments are skewed relative to each other by a first angular displacement, and the first and third segments are skewed relative to each other by a second angular displacement. The first and second angular displacements are selected to cause a net sum of torque ripple produced by each of the segments to be substantially equal to zero during an operation of the motor.
However, when realizing a skewed arrangement of permanent magnets at a mechanical support structure of a rotor assembly for an electro-mechanical transducer, a correct mechanical mounting and alignment of the permanent magnets is challenging and time consuming.
A further disadvantage of skewing permanent magnets is the matter of fact that most skewing patterns produce an unwanted axial force on the rotor assembly. Such an axial force is detrimental to acoustic noise, to the vibrational behavior, and to the lifetime of bearings.
There may be a need for providing a rotor assembly for a permanent magnet electric machine, which can be configured for a mechanically easy manner and which causes a reduced generation of axial forces acting on the rotor assembly when the permanent magnet electric machine is in operation.